Solar supercapacitors for flexible and wearable electronics

Engineers from the University of Glasgow have used layers of graphene and polyurethane to create a flexible supercapacitor that can generate power from the sun and store excess energy for later use — a breakthrough that could lead to a new generation of flexible electronic devices.

Described in the journal Advanced Science, the research is the latest development from the university’s Bendable Electronics and Sensing Technologies (BEST) research group, led by Professor Ravinder Dahiya. He said the group has had “a string of successes … in creating flexible, graphene-based devices which are capable of powering themselves from sunlight”.

“Our previous generation of flexible e-skin needed around 20 nanowatts per square centimetre for its operation, which is so low that we were getting surplus energy even with the lowest-quality photovoltaic cells on the market,” Prof Dahiya said.

“We were keen to see what we could do to capture that extra energy and store it for use at a later time, but we weren’t satisfied with current types of energy storage devices such as batteries to do the job, as they are often heavy, non-flexible, prone to getting hot and slow to charge.”

With this in mind, the researchers developed their own device topped with a touch-sensitive layer of graphene — a highly flexible, transparent ‘super-material’ form of carbon layers just one atom thick. Sunlight which passes through this top layer used to generate power via a layer of flexible photovoltaic cells below. Any surplus power is stored in a newly developed supercapacitor, made from a graphite-polyurethane composite.

The team worked to develop a ratio of graphite to polyurethane which provides a relatively large, electroactive surface area where power-generating chemical reactions can take place, creating an energy-dense flexible supercapacitor which can be charged and discharged very quickly. Similar supercapacitors developed previously have delivered voltages of 1 V or less — making single supercapacitors largely unsuitable for powering many electronic devices — whereas the team’s new supercapacitor can deliver 2.5 V.

The researchers demonstrated the effectiveness of their new material by powering a series of devices, including a string of 84 power-hungry LEDs and the high-torque motors in a prosthetic hand, allowing it to grasp a series of objects. In laboratory tests, the supercapacitor has been powered, discharged and powered again 15,000 times with no significant loss in its ability to store the power it generates.

Prof Dahiya said the device takes the researchers “some distance towards our ultimate goal of creating entirely self-sufficient flexible, solar-powered devices which can store the power they generate”.

“There’s huge potential for devices such as prosthetics, wearable health monitors and electric vehicles which incorporate this technology, and we’re keen to continue refining and improving the breakthroughs we’ve made already in this field,” he said.

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